1. Field of the Invention
This invention relates to medical imaging systems such as magnetic resonance, X-ray and ultrasound scanners, and more particularly to imaging systems designed to provide images of lumens within the body.
2. Discussion of Prior Art
Several methods are currently available for the in-vivo imaging of vessels and other lumens within the body. These include X-ray angiography, MR angiography and ultrasonic imaging. When X-ray procedures are employed to image lumens within the body, an X-ray opaque substance must be introduced into the lumen. For X-ray angiography, an iodinated contrast agent is typically injected into the bloodstream. For imaging the colon, on the other hand, a solution containing a Barium salt is frequently introduced into the patient. These contrast agents permit the visualization of the shape of the lumen by providing visual contrast between the inside of the lumen (which absorbs the X-rays) and the surrounding tissue (which is transparent to X-rays). Undesirable aspects of X-ray methods include the use of ionizing radiation, the use of toxic contrast agents and the difficulty of acquiring three-dimensional information without using a Computed Axial Tomography (CAT) scanner.
Magnetic resonance (MR) can also be used to make images of lumens within the body. Image contrast can be based upon velocity-induced phase shifts (as in phase-contrast MR angiography) or upon differences in T.sub.1 caused by the injection of a T.sub.1 relaxation agent. While MR imaging has the potential to discriminate between different types of lesions in a lumen wall and in many situations can be used to make diagnostic quality angiograms, the inherent low signal-to-noise ratio of MR imaging limits its spatial resolution.
In some parts of the body, ultrasonic imaging can be used to determine the shape of a lumen with a greater resolution that that available with magnetic resonance. Ultrasonic imaging can be acquired from outside the body using a hand-held probe applied next to the skin, or from inside the body using an ultrasonic imaging catheter. In both forms of ultrasonic imaging, the probe position and orientation are manipulated by the operator to maximize image quality and utility. Unfortunately, the exact position and orientation of the probe is not easily incorporated into the ultrasound image, since images are typically obtained without reference to an external or anatomical landmark.
Operator dependency is particularly severe when a vascular ultrasound probe is used. In these procedures, the probe is not held by the operator. Instead, the probe is placed at the end of a catheter which is inserted into a blood vessel. The catheter is manipulated by the operator, but must be followed with an X-ray fluoroscope to insure proper placement and orientation. The ultrasonic images collected by the probe is typically a cross-section of the vessel, but since the orientation of the probe can only be known with the X-ray fluoroscopic image, the ultrasonic image by itself cannot be used to provide information regarding the larger features of the vessel.
One alternative to using an X-ray image to locate an ultrasound catheter is to monitor the insertion depth of the catheter. This approach permits the reconstruction of data along the length of the vessel. Since no information is obtained about the orientation of the catheter within the vessel, however, the vessel can not be properly reconstructed into an image which shows the vessel's curvature and morphology. Reconstruction of ultrasonic images into larger data sets in which insertion depth is exclusively used to provide spatial information will be inherently and irreversibly distorted, particularly in regions of vessel curvature.
Several methods exist to follow the location of an invasive device within the body. These methods include MR tracking as disclosed in "Tracking System and Pulse Sequences to Monitor the Position of a Device Using Magnetic Resonance", C. L. Dumoulin, S. P. Souza and R. D. Darrow (U.S. Pat. No. 5,307,808) and radio frequency tracking as disclosed in "Tracking System to Follow the Position and Orientation of a Device Using Radio-Frequency Fields", C. L. Dumoulin, J. F. Schenck, and P. B. Roemer (U.S. Pat. No. 5,377,678). While these methods provide an instantaneous measurement of device location, they are not able to provide information about the diameter of a lumen.
What is needed is a means for acquiring high resolution images of luminal features such as location and wall composition within the body.